The present invention relates to a single wire steel cord used for reinforcing various vehicle tires and a method of manufacturing the same, particularly, to a single wire steel cord buried in a tire belt portion of a tire and a method of manufacturing the same.
In recent years, the total amount of an exhaust gas is severely regulated as a measure for promoting the program for suppressing the temperature elevation of the earth. In this connection, the reduction in the fuel consumption of the vehicle is being vigorously studied, and an extensive research is being made on the reduction in the thickness of the rubber portion of the tire in an attempt to decrease the weight of the tire. Under the circumstances, the vehicle industries are ardently looking forward to the development of a steel cord as a reinforcing material of the tire. The development of a single wire steel cord, which is expected to make the tire thin to decrease the tire weight and to improve the tire performance while the vehicle is run, particularly, the cornering power and comfortableness of the riding, is of high importance in the future in view of the improvement of the environment of the earth.
The belt layer of a radial tire for a motorcar for riding is formed between the tread and the carcass and acts as a belt stretched in the circumferential direction so as to strongly fasten the carcass like a hoop and, thus, to improve the rigidity of the tread. The particular function of the belt layer is indispensable in order to enable the tire to support the weight of the vehicle and to exhibit its cornering power.
A steel cord of (1.times.n) stranded structure, which is prepared by stranding a plurality of wires, is generally used in the tire belt layer. The steel cord of the wire-stranded structure, which certainly exhibits a high rigidity, causes the tire to exhibit an unduly large repulsive force when the vehicle runs on an unpaved road having a rough road surface, leading to a poor riding comfortableness, and also causes the tread surface of the tire to tend to be cracked. Where the tread surface is cracked, the rain water or the like is likely to enter the tire through the cracks so as to promote corrosion of the cord wire. Further, if the tire is deformed or vibrated, the stranded wires are rubbed each other to cause a so-called "fretting wear", with the result that the cord wire is markedly deteriorated by fatigue.
In order to overcome the above-noted problems, it is proposed to use a single wire steel cord made of a round wire, i.e., a wire having a circular cross section, in the belt layer of the tire in place of the steel cord of the stranded structure, because the single wire steel cord is superior in flexibility to the steel cord of the stranded structure.
However, the conventional single wire steel cord made of a round wire and the steel cord of the (1.times.n) stranded structure give rise to serious problems as pointed out below.
The characteristics used for evaluating the performance of the steel cord include a "kill" (i.e., torsion) and an arc height. The kill is one of the cord characteristics used for evaluating the rotary torque inherent in the cord itself. On the other hand, the arc height is one of the cord characteristics used for evaluating linearity of the steel cord. If the kill is non-uniform or the arc height is unduly large, a calendered sheet is twisted or swollen in the calendering step included in the tire manufacturing process, in which steel cords are arranged on a thin rubber sheet, followed by covering the arranged steel cords with another thin rubber sheet such that the steel cords are sandwiched between the two rubber sheets.
In the conventional cord of (1.times.n) stranded structure or a single round wire cord, the kill and the arc height tend to be varied by the material factor of the cord and by the mechanical factor such as the wire drawing machine or a stranding machine. Particularly, the round wire cord gives rise to a large variation in the kill, making it necessary to apply inspection to each steel cord product even in the general quality guarantee level.
(I) Kill (i.e., Torsion):
The evaluation of the rotary torque (kill) inherent in the steel cord itself is particularly important as a performance of the steel cord used in the tire. The kill will now be described with reference to FIG. 1.
For measuring the kill, a cord 2 is withdrawn from a product cord spool 1 in a length L (=6m), and a cord terminal portion 2c is folded in an L-shape and fixed by a fixing tool (not shown). Then, the cord terminal portion 2c is released from the fixing tool. As a result, the cord terminal portion 2c is rotated about the axis of the cord depending on the degree of twisting of the cord 2. The number of rotations is counted to determine the kill. In the case of the ordinary S-stranding, the rotation in the direction equal to the stranding direction, i.e., clockwise direction, is called a positive kill (+), and the rotation in the opposite direction, i.e., counterclockwise direction, is called a negative kill (-). In general, the kill is satisfactory if the number of rotations is within a range of .+-.2 rotations, and the cord having a kill of this level gives rise to no practical problem.
(II) Arc Height:
The evaluation of the linearity of the steel cord under a non-constrained state (arc height) is important as a performance of a steel cord used in a tire belt portion. The arc height, which is denoted by "AH" in FIG. 2B, is the height of the arc made by a cord 2 cut to have a length L2 (=400 mm) as shown in FIG. 2A, with the both ends of the cord 2 kept in contact with a flat plate 3. In general, the steel cord having an arc height AH of 30 mm or less is satisfactory and gives rise to no practical problem.
(III) Cornering Power and Comfortableness of Riding:
A good operability of the steering wheel during the high speed running of the vehicle, i.e., a large cornering power for avoiding danger, is one of the performances required for a steel radial tire. A steel cord having a high rigidity is used in the conventional steel radial tire. However, when the vehicle runs on an unpaved rough road, the vertical vibration received by the tire is directly transmitted to the driver, leading to a poor comfortableness of riding. The performances required for the reinforcing steel cord for the tire belt layer are the durability in the lateral direction in the cornering step and the comfortableness of riding during running of the vehicle.
It is important for the steel cord in the future to achieve these two performances.
(IV) Thinning of Tire Rubber Layer:
In recent years, vehicles tend to be designed to suppress the fuel consumption as a measure against the temperature elevation of the earth. In this connection, it is important to reduce the weight of the tire. However, it is difficult to decrease sufficiently the thickness of the tire rubber layer in the conventional tire using a stranded steel cord or a single wire cord made of a round steel wire.